The present invention relates generally to applied optics, spectra-chemical image processing, chemical identification, and chemical analysis. More specifically, the present invention relates to apparatus and methods for the non-destructive testing and identification of the composition of a sample of plastics or other materials using Raman spectroscopy and computerized signal processing. The present invention particularly relates to apparatus for identifying the composition of various layers of a material beyond merely the surface layer.
A widespread need exists in industry and government for identifying materials. For example, automobile companies and plastics manufactures must identify and separate plastic resins in recycling operations. Pharmaceutical companies must monitor chemical constituents during drug production. Monitoring agencies and firms must assay waste stream flows into the environment. Law enforcement agencies must identify the presence of illicit drugs in the field in order to combat criminal drug trafficking.
In today's environmentally conscious society, simple economics provides a strong incentive for manufacturers to minimize their use of natural resources and public landfills. Substantial economic benefit can be gained by turning to recycling as a source for raw materials and as the ultimate repository for the manufactured goods. Using recycled raw materials in products can increase profits by saving materials costs and energy. Efficiently recycling packaging and production waste can save landfill charges and provide a cost recovery stream. Further, manufacturing goods with recycled content, and designing goods that themselves are recyclable, is a civic duty that also offers public relations benefits that are worthwhile from a marketing standpoint.
Many suppliers, however, face difficulties in using recycled feed streams.
All companies face competition, and in the marketplace, price alone does not guarantee market share. Most manufacturers value quality and consistency of goods more than the abstract notions of civic duty and environmental policy.
Further, many production lines operate with just-in-time inventories, in which a factory receives all the components necessary to assemble a product only hours before they are needed. The presence of only a few defective parts can shut down a production line until replacement parts arrive. Such a shut down can cost a manufacturer many thousands of dollars.
Thus, it is easy to understand why companies have been reluctant to include recycled materials in products. Recycled materials must have documented histories so that they are assured of compatibility with the manufacturing process. A misidentified piece of recycled material included with virgin material can destroy an entire production run. Maintaining the history of recycled goods, or even knowing their exact composition is difficult, if not impossible, with current technology.
Millions of tons of plastic and other materials are deposited in landfills or incinerated every year due almost solely to the lack of sufficient technology to avoid cross contamination between different types of plastic or other material during collection. The need therefore exists for an effective, economical means to identify a variety of materials, and specifically plastics, on site in scrap yards, warehouses, factories and recycling centers. The successful commercialization of an instrument with such capabilities would greatly increase the recycling rates for plastics and perhaps many other materials. By offering a simple means to overcome the difficult problem of material identification, the present invention seeks to help make manufacturers more receptive to including recycled content in their products, and purchasers more confident of the quality of those products.
Many methods exist for identifying materials. One test for plastic materials, for example, involves the burning of a small sample of the plastic material. Upon smelling the smoke, a trained technician can identify several different classes of plastics with reasonable success. While this method can be employed in a laboratory, such methods are not appropriate or practical for commercial or production line applications. This type of chemical analysis would also not be acceptable to law enforcement personnel or the courts for the identification of Cocaine.
An assortment of analytical identification methods exist, such as Fourier Transform Infrared Spectroscopy (FTIR) and X-ray fluorescence (XRF), for the non-destructive testing of materials. An example of FTIR technology is disclosed in U.S. Pat. Nos. 5,510,619. While well known and used, the FTIR process is not practical in many commercial applications because the method is very sensitive to dirt, surface roughness, coatings, moisture, and sample motion during identification. The XRF process is also used but it is relatively expensive. Other analytical identification methods are disclosed, for example, in U.S. Pat. Nos. 5,256,880 and 5,512,752.
Raman spectroscopy, discovered by C. D. Raman in 1928, has many unique qualities that can be advantageously employed in the practical identification of materials. Raman signals, generated by the interaction of monochromatic light and a sample, are not affected by dirt, surface finish, coatings, or any motion of the sample being identified. Significantly, Raman signals are also not as sensitive to water, glass or quartz as other infrared signals. As a result, chemical samples can be contained within a glass vessel, or even suspended in an aqueous solution without affecting the Raman signal. The Raman process also has a significantly higher working distance with a controllable depth of field than other processes and can "look through" a container to the chemical sample contained inside. In many instances, however, the Raman signal from the glass is very strong as compared to the chemical sample contained within the glass thus presenting a significant signal to noise ratio problem that must be solved in order for accurate identification of the chemical sample contained within the glass container.
Despite these advantages, Raman spectroscopy is not widely used because of a low signal to noise ratio inherent in Raman Spectroscopy. Traditionally, the excitation light source, typically a laser, is directed continuously against a chemical sample, and the Raman signal is collected over time. An example of such an apparatus is disclosed in U.S. Pat. No. 5,534,997. Increasing the power of the excitation laser in the Raman process can increase the strength of the Raman signal and reduce the required sampling time. However, the increase in power can cause thermal damage to the sample particularly if the sample has low thermal conductivity that is typical of plastics. The increase in power can also cause "black body" or thermal radiation that can overwhelm the Raman signal. It is commonly assumed, therefore, that Raman spectroscopy is not appropriate for the practical identification of highly energy absorbent materials such as black or highly pigmented plastics. In the extreme case, such highly absorbent materials can char or burn thus rendering the material unsuitable for further use. As an alternative, optical concentrators such as are disclosed in U.S. Pat. No. 5,615,673 can be employed to gather the Raman signal over a wider range of angles, but physical limitations suggest that the signal is unlikely to be enhanced by more than a factor of about 3, and any masking signal will be enhanced by a similar factor leading to no improvement in signal to noise ratio.
Another challenge is presented by plastics that are in situated in layers and the identity of the plastic in question may be other than the first layer. In such circumstances it is desirable to recover the Raman signal from only the layer of interest. For example, many plastic containers are today made of five or seven co-extruded layers of various plastics that may be necessary to identify to determine initial product quality or whether recycling is possible. Another example is found in automobile windshields and other "safety glass" in which two layers of glass are bound together by a layer of plastic, such as a polyvinyl butyral resin (PVB), of various chemical compositional variation which can be important to the identification of source, quality, etc. In many instances, it is desirable to determine the composition of the inside layers of such articles without physically directly accessing the layer or layers in question.
What is needed is a system for analyzing and identifying the composition of a wide variety of materials that is fast enough for practical application in a commercial setting, insensitive to sample impurities or surface imperfections, tolerant of water and common sample containers, and which does not damage the sample being analyzed and identified.